Progressive Collapse Scenario Research Articles

One-Sided Steel Shear Connections in Progressive Collapse Scenario

One-Sided Steel Shear Connections in Progressive Collapse Scenario

Experimental study of precast dry connections constructed away from beam–column junction under progressive collapse scenario

This paper presents experimental studies conducted on precast dry connections under progressive collapse scenario. Reduced one-third-scale precast beam–column assemblies are constructed as test specimen by providing connection away from beam–column junction. Dimensions of test specimens are extracted from 6-story symmetrical building having overall plan dimensions of 16 m × 12 m. Each specimen consists of two span beams and three columns with removed middle column, which represents progressive collapse scenario. Dry connection between precast elements were constructed by adopting two different types of detailing such as welding and bolting of steel plates at connection region. Monotonic vertical load was applied at the location of removed middle column with the help of hydraulic jack. Behavior of test specimens was measured in terms of ultimate load carrying capacity, deflection profile along the length, failure pattern and crack propagation. The performance of precast specimens having dry connection was also compared with that of monolithic connection. From the results of experimental studies, it was observed that precast dry connection with welded plates was having maximum ultimate load carrying capacity, while the same for precast dry connection with bolted plates was lesser than that of monolithic connection. However, performance of precast dry connection with bolted plates can be enhanced by ensuring appropriate quality of concreting around connection region. Based on the study, it was concluded that building having precast elements connected away from the beam–column junction with adequate connection detailing behaves similar to that of monolithic construction and can be a sustainable alternative of cast-in-place construction.

Component tests and numerical simulations of composite floor systems under progressive collapse

This paper presents experimental and numerical studies on the behaviour of components and composite floor system under column removal scenarios. Shear connectors, double angle-cleat connections and flush end plate connections were extracted from a three-dimensional composite floor system and tested under different loading conditions. The load-displacement curves of these components were compared with the results predicted from existing models. A simplified numerical model was developed to simulate the behaviour of composite floor systems subject to progressive collapse, in which the properties of springs were determined from component tests. The model was then validated against experimental results in terms of load-displacement relationship and failure mode. Finally, parametric studies were conducted to investigate the effects of slab thickness, aspect ratio and reinforcement. Numerical results suggested that the aspect ratio of slabs has the most significant effect on the load capacity of composite floor systems under progressive collapse scenarios.

Experimental Investigation on Vulnerability of Precast RC Beam-column Joints to Progressive Collapse

The multi-story buildings are susceptible to progressive collapse in the event of the removal of one or more columns due to the exposure to blast loads. The lack of structural continuity in precast concrete buildings makes these buildings more vulnerable to progressive collapse as compared to the regular cast-in-situ concrete buildings. This study presents experiments involving two types of detailing of precast beam-column joints using half-scale test specimens when the middle column is suddenly removed. The test specimens represent the most prevalent precast beam-column joints. One conventional cast-in-situ test specimen, having continuous top and bottom beam rebars, was used for comparison. The progressive collapse scenario was simulated by removing the central column support and applying a sudden vertical load on this column at a rate of 100 mm/s until failure. Test results helped in developing better understanding about the progressive collapse potential in the existing precast buildings. This study highlights the need for the rehabilitation of beam-column connections in existing precast buildings and necessitates the need for innovative beam-column connections for improving the progressive collapse resistance.

Progressive collapse behaviour of endplate connections to cold-formed tubular column with novel Slip-Critical Blind Bolts

A novel blind bolt, named Slip-Critical Blind Bolt (SCBB), was recently invented at Tongji University to be used as slip-critical high strength bolt for moment connections with handy installation from outside of tubular column. This paper applied SCBBs in extended endplate connections to cold-formed Square Hollow Section (SHS) columns, and investigated this application in the beam-to-column assembly against progressive collapse. The performance of SCBBs at connection regions was evaluated in four full-scale specimens with different configurations. Four distinct failure modes were identified, including fracture at beam section, pull-out of bolts from column wall, plastic deformation of endplates and welded bottom flange fracture forcing the reinforcing component to work. Throughout the damage evolution, the SCBBs performed satisfactorily by clamping the connected plates firmly without slip occurring at the plate interface, showing a good mechanical property as being slip-critical high strength bolts. Under the progressive collapse scenario, SCBBs maintained the integrity of the blind bolted connection, thus facilitated the assembly to effectively develop the catenary mechanism which provided significant vertical resistance under large deflection. The considerable potential of SCBB to be applied into the blind bolted moment connections against progressive collapse and its effectiveness were further verified by comparison between the test specimens and other published moment-resisting beam-to-column assemblies.

A new type of truss joint for prevention of progressive collapse

This paper presents a study on a new type of truss joint (Pinned-Slidable or PS joint) that is designed to maximize the function of catenary action upon a sudden member removal. The basic idea for a PS joint is such that it is fixed on the bottom chord without sliding under the design load, but can slide along the bottom chord to help distribute the unbalanced tensile forces that the catenary action generates on different bottom chord members. The sliding resistance of this joint is provided by the friction generated by the preloaded bolts and the shear resistance of a locking rod, which can be designed according to the proposed design schemes. A Warren truss with PS joints as the bottom joints (Truss-PSJ) has been designed and tested under a scenario of a sudden removal of one of its diagonal members, and the response of the structure is compared with that of an identical truss model except for its non-slidable joints (truss-PJ). Results show that significant catenary action develops in the bottom chord of the remaining structure in the process of establishing a new equilibrium state after the member removal, and the catenary action tends to generate much larger tensile forces in the bottom chord members in the mid-span than in the neighboring bottom chord members, leading to the sliding of a PS joint. By comparing the test results of truss-PSJ with those of its counterpart truss-PJ, the benefits of the PS joints have been clearly demonstrated. The sliding of the PS joint not only releases the excessive unbalanced forces between neighboring members and thus enables a fuller development of the catenary action, but also facilitates the adaptation of the remaining structure towards a new balanced state with a near-optimal deformation shape. A finite element analysis has also been conducted to confirm the above experimental observations, and to demonstrate the performance of PS joints under other progressive collapse scenarios. Furthermore, a method for determining the sliding resistance of PS joints is also proposed.

Strengthening of precast RC beam-column connections for progressive collapse mitigation using bolted steel plates

Being one of the most critical scenarios in extreme events such as blast attacks, the progressive collapse of reinforced concrete (RC) structures has attracted the attention of structural engineering community. As precast concrete buildings are deficient in structural continuity, they are more vulnerable to progressive collapse than cast-in-situ RC buildings. Hence, effective rehabilitation techniques to upgrade beam-column joints in existing precast RC buildings for progressive collapse mitigation are needed. The goal of this study is to investigate the effectiveness of using bolted steel plates on the behavior of precast beam-column connections under sudden column-loss scenario. This study presents experiments involving one half-scale precast RC beam-column assembly, which represented the most prevalent types of existing precast beam-column joints in Saudi Arabia. One monolithic test specimen having continuity of top and bottom beam rebars was used for the sake of comparison. Another precast specimen similar to the control one was strengthened using bolted steel plates within the connection region. The progressive collapse scenario was simulated by removing the central column support and applying a sudden vertical load on this column at a rate of 100 mm/s until failure. The collapse load of both monolithic and strengthened specimens was predicted using a simplified section analysis procedure. The analysis was then used for some useful parametric studies in which the effect of different steel plate parameters on the response of test frames under middle column-loss scenario was investigated.

Robustness analysis of steel structures with various lateral load resisting systems under the seismic progressive collapse

Abstract Robustness is called to non-sensitivity of a structure to local damage. In the literature of Civil Engineering, the sensitivity of structures to the local damage which can cause a progressive collapse is called Robustness Index. After 9/11, progressive collapse drew the attention of researchers and since that time, increasing the Robustness Index in a structure has been considered as one of the factors in strengthening the structure against progressive collapse. The aim of this study is assessing the Robustness Indices of steel structures having various lateral load resisting systems. So, 8-story, 4-story and 15-story structures with Moment Frames, Concentrically Braced Frames and Eccentrically Braced Frames were modeled and assessed under Robustness analysis. For a progressive collapse analysis, the Alternative Load Path method was used and after emerging local damage, the potential of progressive collapse was assessed using the Robustness Index. First, Robustness analysis was carried out on the basis of Robustness Index assessment using Stiffness and Base Shear methods. Subsequently, it continued by introducing a simple method based on Energy. The results show that despite their simplicity, the methods which are based on Stiffness and the Base Shear, have lots of disadvantages. But, the Energy method has better capabilities for understanding the behavior of structure under seismic loads in progressive collapse scenario. Furthermore, structures which have bracing had a better reaction in progressive collapse scenario.

08.51: An experimental study of composite beam‐column joints subject to impact loads

ABSTRACTBeam‐column joints are one of the key elements in steel and composite frames. Till now, many research studies have focused on the behaviour of bare steel beam‐column joints and the influence of composite slabs subjected to quasi‐static loads. In this study, the research was extended to dynamic loading condition. Two beam‐column joint specimens with composite slabs, which were designed based on Eurocode 4, were tested using an MTS drop‐weight machine. This loading condition is probably more realistic for joints under progressive collapse scenario, which is a dynamic process. One of the specimens was a middle joint subject to sagging moment. The other specimen was a side joint subject to hogging moment. Behaviour of both joint specimens, including impact force, displacement of the middle column, strain and failure mode, was presented. Differences in the behaviour of the two joint specimens were observed and explained.

Evaluation of Collapse Distribution in Three-Story RC Moment-Resisting Asymmetric Buildings due to Earthquake Loads

Among important issues in progressive collapse behavior of a building is tracking down the type and location of the damaged elements. This paper deals with the distribution of 3D collapse from the first element to the entire symmetric and asymmetric RC buildings due to earthquake loads. The variables of such analyses are earthquake load intensity and the value of one directional mass eccentricity. Results show that the collapse propagation is dependent on the degree of asymmetry in building. Some patterns are also determined to predict the progressive collapse scenarios in similar symmetric and asymmetric buildings. Patterns show that the propagation of collapse is horizontal through the stories, but not vertical through the height of the buildings. Spread of the collapse is independent of the earthquake records and damage concentration is larger in places with larger mass concentration.

Collapse resistance assessment through the implementation of progressive damage in finite element codes

Different procedures for assessing the robustness of a reinforced concrete (RC) frame under progressive damage are proposed and compared. The removal of a column in a RC frame structure is modeled with a commercial nonlinear finite element software according to three alternative strategies: (i) reduction of mechanical properties of the damaged column, (ii) incremental loading of the structure after total removal of the damaged column, and (iii) incremental unloading of internal forces on the damaged column. Nonlinear analysis is performed under a prescribed load combination on three RC frames designed with three Italian building codes in force in different periods. Despite the differences in the strategies for damage modeling, similarities between structural response predictions are highlighted. In addition, it is shown that seismic design provisions for RC structures increase the ductility of the structure but do not necessarily guarantee robustness to progressive collapse scenarios.

Bond–slip behaviour of deformed reinforcing bars embedded in well-confined concrete

Precast concrete beam–column sub-assemblages may exhibit pull-out failure of reinforcing bars embedded in the middle joint when subjected to column removal scenarios. This failure mode has to be considered in analytical study through a component-based joint model. However, current bond–slip models incorporated in design guidelines are incapable of predicting the load capacity and potential failure mode of reinforcing bars with inadequate embedment length in beam–column joints. Therefore, it is necessary to re-evaluate the bond–slip behaviour of embedded reinforcement. This paper presents an analytical approach to predict the force–slip relationship of reinforcement. In the approach, bond stress at the post-yield stage of reinforcement is calculated from existing test data of deformed rebars embedded in well-confined concrete under pull-out loads. The approach is calibrated by experimental results of steel bars with various embedment lengths. Simplified approaches are also developed in accordance with the bond stress profile along the embedment length and the average bond stress. Finally, a component-based joint model is established for precast concrete beam–column sub-assemblages, in which the force–slip relationships of reinforcement are derived from the proposed approach. The joint model yields reasonably good predictions of the compressive arch action and catenary action capacities of sub-assemblages under progressive collapse scenarios.

Behavior of wet precast beam column connections under progressive collapse scenario: an experimental study

Progressive collapse denotes a failure of a major portion of a structure that has been initiated by failure of a relatively small part of the structure such as failure of any vertical load carrying element (typically columns). Failure of large part of any structure will results into substantial loss of human lives and natural resources. Therefore, it is important to prevent progressive collapse which is also known as disproportionate collapse. Nowadays, there is an increasing trend toward construction of buildings using precast concrete. In precast concrete construction, all the components of structures are produced in controlled environment and they are being transported to the site. At site such individual components are connected appropriately. Connections are the most critical elements of any precast structure, because in past major collapse of precast structure took place because of connection failure. In this study, behavior of three different 1/3rd scaled wet precast beam column connections under progressive collapse scenario are studied and its performance is compared with monolithic connection. Precast connections are constructed by adopting different connection detailing at the junction by considering reinforced concrete corbel for two specimens and steel billet for one specimen. Performance of specimen is evaluated on the basis of ultimate load carrying capacity, maximum deflection and deflection measured along the span of the beam. From the results, it is observed that load carrying capacity and ductility of precast connections considered in this study are more than that of monolithic connections.

Seismic collapse propagation in 6-story RC regular and irregular buildings

One of the most important issues in progressive collapse mechanism of the buildings is evaluation of the collapse distribution in presence of the earthquake loads. Here, collapse propagation is investigated by tracking down the location and type of the collapsed beam and column elements, from the first element to the entire buildings. 6-story reinforced concrete ordinary moment resisting frame buildings with one directional mass eccentricity of 0%, 5%, 15% and 25% are studied to investigate differences among the progressive collapse mechanism of the regular and irregular buildings. According to the results of the nonlinear time history analyses, there are some patterns to predict progressive collapse scenarios in beam and column elements of the similar regular and irregular buildings. Results also show that collapse distribution patterns are approximately independent of the earthquake records.

An efficient compound-element for potential progressive collapse analysis of steel frames with semi-rigid connections

In this paper, the formulation of a novel 1D frame compound-element for the materially and geometrically non-linear analysis of steel frames with flexible connections is outlined. The element is formulated based on the force interpolation concept and the total secant stiffness approach, and implemented in a FORTRAN computer code. The accuracy and efficiency of the formulation are verified through some numerical examples. For steel frames with bolted flush end-plate and extended end-plate connections, a static and dynamic progressive collapse assessment based on the alternate load path (ALP) method is undertaken by employing the developed analytical tool and dynamic load factor (DLF) is estimated. Furthermore, the implications of analyzing semi-rigid steel frames based on the assumption of fixed connections and the effects of the connection details on the global response of a frame during different progressive collapse scenarios are investigated.